In an unusual case of close cooperation between the owners of land and leasehold, Waterman Interests has signed a new, 75-year master lease at 400 Park Ave., at 54th Street.

Tod Waterman’s company made the deal with ground owner Benenson Capital Partners, the Benenson family’s operating company, which has owned the site since 1971.

Waterman and institutional investors advised by JP Morgan Asset Management bought the leasehold on the 270,000 square-foot property from RFR in 2010 for $35 million. At the time, the leasehold had only 17 years remaining.

Terms of the new lease were not disclosed. But both sides said the long-term arrangement will prove mutually beneficial.

It will enable Waterman to accelerate its upgrading of the 1957 tower, which was once best known as the home of discount retailer Syms.

When Syms went belly-up, Waterman repositioned the lower floors for high-end financial firms, and quickly signed a lease with City National Bank for much of the former store space.

The $20 million Waterman put into improvements to the lobby, elevators and windows helped stimulate 100,000 square feet of new leasing.

“Benenson recognized that with our stewardship of the asset, it would make a lot of sense for us to collaborate to put the property on a different path,” Waterman said.

Benenson COO Richard A. Kessler praised Waterman, saying, “Tod was willing to invest capital with or without a lease extension.” He added, “400 Park had always troubled me because it was like a poor cousin in close proximity to iconic buildings such as Seagram and Lever House.”

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Billy Macklowe has nailed down another lease at 386 Park Ave. South, the 270,000-square-foot tower his William Macklowe Co. and partner Principal Real Estate Investors bought last fall for $111.5 million.

The Profero lease ups the tower’s occupancy to around 60 percent. Meanwhile, Macklowe is proceeding with capital improvements to the lobby, lower façade and entrance.

Asking rents are in the mid-$50s. Profero was repped by CBRE’s Sam King and Gerry Miovski. CBRE’s Paul Amrich, Neil King, Ross Zimbalist and Kerry Powers acted for the landlord.

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We still remember when Douglas Durst was telling us of his envisioned 1 Bryant Park — which was then a hole in the ground at Sixth Avenue and 42nd Street — “Our rents will have a ‘1’ in front of them.”

Now, two new $100-plus per square foot leases have brought the 2.1 million square-foot tower, which is home to Bank of America, to a more than 99.9 percent occupancy rate.

Hedge fund QFR, which had 13,000 square feet on the 37th floor, has now added another 20,227 square feet on the same floor.

Cushman & Wakefield’s Jared Horowitz repped the tenant.

Ascot Underwriting Inc. took a 5,764 square-foot prebuilt on the 40th floor, which was repped by CBRE’s Michael Rizzo and Zachary Price.

Durst’s in-house leasing chief Tom Bow repped the landlord on both of these deals.